Oxygen production by adsorption

ABSTRACT

A process for producing an oxygen enriched product includes: (a) providing a gas separation apparatus having at least one bed containing a mixture of at least two different nitrogen selective adsorbents, wherein the at least one bed is free of lithium cations; (b) feeding a feed gas containing oxygen and nitrogen into the gas separation apparatus to contact the at least one bed; and (c) recovering from the gas separation apparatus the oxygen enriched product. The process is preferably performed above ambient temperature and/or in a simplified four-step cycle. The cycle includes: (a) feeding a feed gas containing oxygen into a gas separation apparatus to contact at least one bed of the apparatus with the feed gas, wherein the feed gas is at a temperature above ambient; (b) countercurrently evacuating the at least one bed following the feeding; (c) countercurrently purging the at least one bed with the oxygen enriched product under vacuum; and (d) simultaneously pressurizing the at least one bed with a countercurrent stream of the oxygen enriched product and a cocurrent stream of the feed gas.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to the separation of oxygen fromgas mixtures, such as air. More particularly, the invention relates tothe use of mixed adsorbents in the adsorber, a higher air feedtemperature, and/or a simpler vacuum swing adsorption/pressure swingadsorption (VSA/PSA) process design to separate oxygen from air.

[0002] In numerous chemical processing, refinery, metal production andother industrial applications, purified gas streams are employed for avariety of processing purposes. For example, high purity oxygen is usedin chemical processing, steel mills, paper mills, and in lead and gasproduction operations. Oxygen and nitrogen are produced from air,typically by cryogenic distillation for large size applications. Whilesuch cryogenic processing can be very efficient, particularly whenconducted in large size plants, it nevertheless requires complex andcostly equipment.

[0003] Vacuum swing adsorption/pressure swing adsorption processes havealso been used to separate and purify gases, but the production ofoxygen by the VSA/PSA approach has generally been confined to relativelysmall-sized operations with respect to which the use of cryogenic airseparation may not be economically feasible. Many commonly availableadsorbents, particularly the class of materials known as molecularsieves or zeolites, selectively adsorb nitrogen more strongly thanoxygen, and this preferential adsorption is the basis of a variety ofVSA/PSA processes that have been developed for the separation of air toproduce oxygen and nitrogen product gas.

[0004] In the VSA/PSA process, a feed gas mixture, such as air,containing a more readily adsorbable component and a less readilyadsorbable component, e.g., the nitrogen and oxygen components of air,is passed to the feed end of an adsorbent bed capable of selectivelyadsorbing the more readily adsorbable component at a higher adsorptionpressure. Most of the less readily adsorbable component passes throughthe bed and is recovered from the discharge end of the bed. Thereafter,the bed is depressurized to a lower desorption pressure for desorptionof the more readily adsorbable component, and its removal from the feedend of the bed prior to repressurization with feed gas and or lessreadily adsorbed component, and introduction of fresh feed gas foradsorption as cyclic adsorption-desorption-repressurization operationsare continued in the bed. Such VSA/PSA processing is commonly carriedout in multi-bed systems, with each bed employing the same VSA/PSAprocessing sequence on a cyclic, basis interrelated to the carrying outof such processing sequence in the other beds of the adsorption system.In VSA/PSA systems for the recovery of moderate to high purity oxygen(80-95% O₂) product as the less readily adsorbable component of air,each adsorbent bed will commonly contain an adsorbent material capableof selectively adsorbing nitrogen as the more readily adsorbablecomponent, with the selectively adsorbed nitrogen being subsequentlydesorbed and removed from the bed upon reduction of the pressure of thebed from the higher adsorption pressure level to a lower desorptionpressure level. VSA/PSA systems for the recovery of nitrogen producthave likewise been based on the use of adsorbents that selectivelyadsorb nitrogen from air as the more readily adsorbable componentthereof.

[0005] There are various techniques that exist to separate nitrogen fromoxygen. For instance, U.S. Pat. No. 4,329,158 to Sircar discloses aprocess for the separation of nitrogen from oxygen wherein apretreatment adsorptive separation of water and carbon dioxide isperformed prior to the bulk separation of the major constituents of air.Nitrogen enriched waste gas is utilized from the bulk separation portionof the process to regenerate the pretreatment portion of the process.The bulk separation of nitrogen from oxygen is performed with anelevated temperature adsorption of nitrogen, a desorption of bulkseparation beds to a lower pressure, a purge of the beds with productoxygen after desorption countercurrently and two steps ofrepressurization to elevated pressure first with waste gas which isnitrogen enriched and secondly with product oxygen.

[0006] U.S. Pat. No. 5,882,380 to Sircar describes a single-bed PSAsystem comprising a blower, an adsorber vessel, and a gas productstorage tank that separates a gas mixture using a three-step cyclecomprising adsorption, evacuation, and pressurization used to separatenitrogen from a feed air. Pressurization is accomplished by introducinggas from the gas product storage tank into both the feed end and theproduct end of the adsorber vessel. Preferably a portion of thepressurization gas is introduced into the adsorber vessel by the blower,which also is used for providing feed to the adsorber and forwithdrawing gas from the adsorber during the evacuation step.

[0007] Ackley et al. (European Patent Application No. 0 963 777)discloses a PSA apparatus for the separation of a heavy component from alight component in a feed stream. The apparatus includes an adsorbentbed comprising either a mixture of adsorbents or composite adsorbentparticles wherein each particle comprises two or more adsorbents. Atleast one of the adsorbents is comparatively weak, i.e., NaX, and theother is comparatively strong, i.e., LiX. Another embodiment of theinvention is a PSA prepurifier having a bed of adsorbent material whichcomprises a mixture of adsorbents, or composite of adsorbent particleswherein each particle comprises at least two adsorbents, at least one ofthe adsorbents being comparatively strong, i.e., NaY and at leastanother of the adsorbents being comparatively weak, i.e., activatedaluminum.

[0008] The adsorbent is often the key to the effectiveness of oxygenproduction processes. Therefore, much attention has been given to thedevelopment, improvement and manufacture of adsorbents. For example,specialized zeolite adsorbents have been synthesized through ionexchange, lower Si/Al structures and improved activation procedures.These additional and/or improved manufacturing steps have resulted inhigher costs for these specialized adsorbents (e.g., LiX) compared tomore common adsorbents (e.g., 5A and 13X). In many processes, theadsorbent has become a significant fraction of the overall capitalinvestment. Thus, there is considerable incentive to reduce the cost ofthe adsorbent if doing so results in an overall reduction in the cost ofthe desired product of the separation.

[0009] Accordingly, there is a need for alternative systems forseparating oxygen from gas mixtures, such as air, wherein the systemsoptimize the productivity of relatively inexpensive adsorbents to renderthe systems economically competitive with state of the art systemsemploying more sophisticated but expensive adsorbents.

[0010] All references cited herein are incorporated herein by referencein their entireties.

BRIEF SUMMARY OF THE INVENTION

[0011] The invention provides a process for producing an oxygen enrichedproduct from a feed gas containing oxygen and nitrogen. The processcomprises: (a) providing a gas separation apparatus having at least onebed containing a physical mixture of at least two different nitrogenselective adsorbents, wherein the at least one bed is free of lithiumcations; (b) feeding a feed gas containing oxygen and nitrogen into thegas separation apparatus to contact the at least one bed; and (c)recovering from the gas separation apparatus the oxygen enrichedproduct. The process is preferably performed above ambient temperatureand/or in a simplified four-step VSA/PSA cycle. The cycle includes: (a)feeding a feed gas containing oxygen and nitrogen into a gas separationapparatus to contact at least one bed of the apparatus with the feedgas, wherein the feed gas is at a temperature above ambient, e.g., fromabout 40° C. to about 100° C.; (b) countercurrently evacuating the atleast one bed following the feeding; (c) countercurrently purging the atleast one bed with the oxygen enriched product under vacuum; and (d)simultaneously pressurizing the at least one bed with a countercurrentstream of the oxygen enriched product and a cocurrent stream of the feedgas.

[0012] Also provided are an apparatus for performing the process of theinvention and a process for producing an oxygen enriched product from afeed gas containing oxygen and nitrogen using the four-step cycle andelevated temperature with lithium-containing and/or lithium-free beds.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

[0013] The invention will be described in conjunction with the followingdrawings wherein:

[0014]FIGS. 1, 2 and 3 are graphs of N₂ capacity versus adsorptionpressure.

DETAILED DESCRIPTION OF THE INVENTION

[0015] While various techniques exist to separate nitrogen from oxygen,the inventors are not aware of the use of mixed adsorbents in theadsorber column which completely exclude the use of a lithium exchangecation, and higher feed temperature, or simpler process designs.

[0016] There is a need for a vacuum swing adsorption/pressure swingadsorption (VSA/PSA) system and process that reduces the cost ofadsorption processes. This need can be met by improving adsorbentefficiency and/or by reducing the cost of the adsorbent. Improvedadsorbent efficiency means greater adsorbent effectiveness for thedesired separation.

[0017] While conventional zeolite molecular sieves associated withlithium exhibit a highly desirable capacity and selectivity toward theadsorption of nitrogen from air, it has been found that less costlymixtures of at least two low to moderate nitrogen selective adsorbents,such as zeolites containing monovalent and divalent cations whichinclude calcium, sodium, barium, strontium, magnesium, and cesium, canalso be used for the desired selective adsorption of nitrogen from feedair, and the recovery of oxygen as the desired product gas at a reducedor comparable cost. Other low cost nitrogen selective zeoliticadsorbents suitable for use in the present invention are exchanged X,exchanged A, and mordenite zeolites, such as NaX, 5A, Na-Mordenite, CaXand CaLSX.

[0018] In preferred embodiments, the separation of nitrogen from oxygenis achieved through the use of a mixture of at least two different lowto moderate nitrogen selective adsorbents mixed together. The term“mixture” as used herein means a physical intermixture of ingredients(e.g., a homogeneous blend of adsorbent particles) as opposed to aheterogeneous assembly of ingredients (e.g., layers of differentadsorbents) inside the adsorber. The mixture can be a composite ofadsorbent particles or a mixture of independent adsorbent particles. Themixture preferably contains from about 20% to about 80% by weight of afirst adsorbent and from about 80% to about 20% by weight of a secondabsorbent. The mixture more preferably contains about 50% by weight ofeach of the two adsorbents. It has been found that the use of mixedadsorbents, instead of a single adsorbent, can produce oxygen at acompetitive cost.

[0019] The mixture is preferably free of lithium ion exchanged zeolites,such as LiX. Accordingly, the bed is preferably free of lithium cations.

[0020] Additionally, it has been found that the use of a higher feed airtemperature than ambient, e.g., from about 40° C. to about 100° C., cansignificantly alter the air separation process performance in order toproduce cost-competitive oxygen product using relatively inexpensiveadsorbents. This method can also be used in conjunction with arelatively simple process design (i.e., a four-step cycle), whichresults in a single adsorber, single air blower-vacuum pump combinedsystem which produces oxygen at a competitive cost.

[0021] The invention will be illustrated in more detail with referenceto the following Examples, but it should be understood that the presentinvention is not deemed to be limited thereto.

EXAMPLES

[0022] Simulations were performed to study the cost of O₂ production byVSA using pure gas adsorption data for N₂ and O₂ on various zeolites.The relative costs were developed for a 60 TPD contained O₂ unitproducing 90-92% O₂ at a pressure of 10 psig. The inventors evaluatedthe process performance of (a) several single zeolites (NaX, CaX, 5A,Na-mordenite, CaLSX and LiX), (b) several mixed zeolites (LiX+NaX,CaX+NaX) where an intimate mixture of zeolites at the particle level(not layered) was used, and (c) operating the VSA-O₂ process using a hotfeed air in order to raise the average adsorbent temperature during theprocess steps. The commonly used eight-step VSA O₂ process (minimum twobeds) as well as a simpler four-step VSA-O₂ process using a single bedand a single air blower/vacuum pump combine were evaluated.

[0023] As a result of this evaluation, it was determined that LiXzeolite offers the best performance for O₂ production under commonlyused VSA-O₂ process design conditions (highest O₂ productivity andlowest power) due to its high N₂ working capacity and selectivity andlow N₂ Henry's Law constant and that the performance advantage of LiXzeolite is compromised by its relatively higher cost. All other zeolitesyielded competitive O₂ product costs when used with the commonly usedVSA-O₂ process because of their relatively lower costs. It was alsodetermined that a mixed adsorbent system allows alteration of effectiveN₂ and O₂ adsorption characteristics by design, which in turn provideslarge flexibility in process performance. These systems are also costcompetitive with LiX based systems.

[0024] Table 1 illustrates examples of the adsorptive properties ofvarious ion- exchanged zeolites of different framework structures(commercially developed) used for practical air separation processes.High nitrogen working capacity, high nitrogen selectivity over oxygen,and low heat of adsorption of nitrogen generally lower the specificadsorbent inventory for a given oxygen production capacity and givehigher oxygen recovery from the feed air. Lower Henry's Law constantsfor nitrogen and lower heats of adsorption for N₂ generally reduce theevacuation power for desorption of N₂ in a VSA-O₂ process.

[0025] Table 1 lists adsorbents in chronological order of development ofzeolites for air separation. The N₂ working capacity increases and thenlevels off, the selectivity of adsorption of N₂ over O₂ increases andthen levels off, and the heat of adsorption of N₂ increases and thenlevels off. On the other hand, the Henry's Law constant for N₂ increasesand then decreases for LiX. Thus, LiX is currently the preferredmaterial for production of O₂ because it provides the most desiredproperties. However, LiX is also the most expensive adsorbent listed.TABLE 1 Ex- Rela- N₂ N₂ N₂ Henry's am- tive Working N₂/O₂ Isosteric Lawple Ad- Cost Capacity Select- Heat Constant No. sorbent S/lb moles/kgivity Kcal/mole moles/kg/atm 1 NaX 1.00 0.110 2.9 4.3 0.297 2 5A 1.070.170 3.8 5.7 0.522 3 Na- 2.67 0.216 4.0 6.2 0.982 Mor- den- ite 4 CaX1.27 0.252 10.9 7.1 3.606 5 CaLSX 2.33 0.387 12.7 6.9 4.033 6 LiX 3.700.342 10.2 6.7 1.982

[0026] The performance of a commonly used VSA-O₂ process for productionof 90-92% oxygen from ambient air using the zeolite adsorbents of Table1 was simulated using experimentally measured N₂ and O₂ adsorptioncharacteristics on these materials. The commonly used VSA-O₂ processconsisted of eight cyclic steps: (a) feed air flow with O₂ enrichedproduct withdrawal, (b) feed air flow to provide O₂ enriched purge gas,(c) cocurrent depressurization to provide purge gas, (d) cocurrentdepressurization to provide pressurization gas, (e) countercurrentevacuation, (f) countercurrent purge with O₂ enriched gas under vacuum,(g) countercurrent pressurization with O₂ enriched gas and (h) cocurrentfeed pressurization.

[0027] Table 2 shows the simulated separation performances for differentzeolites. It may be seen that the O₂ productivity (mlb moles of O₂produced/lb of zeolite/cycle) and the specific power for the product(KW/TPD contained O₂) substantially vary from zeolite to zeolite, butthe relative costs of oxygen product ($/ton) is very insensitive to thechoice of the adsorbents.

[0028] The results in Table 2 illustrate that the LiX zeolite has thehighest oxygen production capacity and the lowest power requirement, butits higher price compromises these apparent advantages to give similaroxygen cost as other zeolites. Also, as evidenced by Table 2, othermaterials, such as NaX or CaX, compete well with LiX because they areless expensive than LiX. TABLE 2 Ex- O₂ Relative am- Productivity O₂Specific ple Ad- (mlb) Recovery Power* Relative O₂ No. sorbentmoles/lb/cycle) (%) (KW/TPDc) Cost 1 LiX 0.0597 70.0 1.00 1.00 2 NaX0.0304 52.5 1.18 1.04 3 CaX 0.0371 59.4 1.18 1.03 4 CaLSX 0.0495 64.81.09 1.01

[0029] The inventors noted that even though N₂ adsorption isotherms arestrikingly different on these zeolites (as shown by FIG. 1), suchdisparity in isotherm shapes does not create any meaningful differencein the O₂ production cost using today's zeolite cost structure.

[0030] The above unexpected results led the inventors to invent (a)several different ways of altering the N₂ and O₂ adsorption isothermshapes and thereby obtain O₂ production costs which are competitive withthe current cost for the commonly used VSA-O₂ process using the LiXadsorbent, and (b) several different ways of operating simpler and lesscapital intensive VSA-O₂ processes than the present complex VSA processin order to match the current cost of O₂ production.

[0031] First, the inventors discovered that the effective shapes of N₂and O₂ adsorption isotherms can be altered by design using a packed bedof intimately mixed (not layered) particles of two or more differentzeolites. This will introduce a synthetic adsorbent heterogeneity andchange the effective adsorption isotherms, N₂ selectivities over O₂ andheats of adsorption for the air separation process. For example, thepure gas adsorption isotherms of a composite adsorbent bed will be givenby the weighted averages [(weight % of a type)×(adsorption capacity ofthat type)] of the isotherms of each type of adsorbent present in themixture.

[0032]FIG. 2 shows the composite N₂ isotherms for 50% LiX+50% NaX and50% CaX+50% NaX mixtures. As illustrated in FIG. 2, drastic changes inisotherm shapes can be created (compared to single adsorbents) by mixingdifferent adsorbents. It was also observed that the corresponding unitcost of the mixed adsorbent is significantly less than that of singleadsorbents.

[0033] Table 3 illustrates the simulated performance of a commonly usedVSA-O₂ process using the above described mixed adsorbents. The operatingconditions of the processes are the same as those used for generatingthe data of Table 2. It may be seen that even though the mixed gasadsorbents exhibit lower O₂ productivity and higher specific power, theO₂ production cost is comparable with that of LiX due to the lessexpensive cost of NaX and CaX zeolites. TABLE 3 O₂ Ex- Productivity O₂Relative am- (mlbmoles/ Recov- Specific ple lb/ ery Power* Relative O₂No. Adsorbent cycle) (%) (KW/TPDc) Cost 1 LIX 0.0597 70.0 1.00 1.00 250% LiX + 0.0491 65.0 1.02 0.99 50% NaX 3 50% CaX + 0.0353 57.3 1.161.02 50% NaX

[0034] Secondly, the inventors have discovered that the shapes of the N₂and O₂ adsorption isotherms can also be altered by changing theeffective adsorbent temperature within the adsorbers during the airseparation process steps. This can be achieved by increasing the feedair temperature above ambient, e.g. from about 40° C. to about 100° C.

[0035]FIG. 3 shows the N₂ adsorption isotherm of CaLSX zeolite at 140°F. (60° C.) and compares it with the N₂ isotherm of LiX at 30° C. Asillustrated by FIG. 3, the Henry's Law constant of CaLSX becomes similarto that of LiX (30° C.) at 60° C. Table 4 demonstrates the performanceof a commonly used VSA-O₂ process at 140° F. (60° C.) feed temperatureusing CaLSX. CaLSX is a highly nitrogen selective binderless exchange Xzeolite adsorbent. At a temperature above ambient, e.g., from about 40°C. to about 100° C., the inventors discovered that the processperformance and O₂ costs of CaLSX are comparable with those for LiX.TABLE 4 O₂ Relative Feed Gas Productivity O₂ Specific ExampleTemperature (mlbmoles/lb/ Recovery Power* Relative No. Adsorbent (° F.)cycle) (%) (KW/TPDc) O₂ Cost 1 LiX 97.0 0.0597 70.0 1.00 1.00 2 CaLSX140.0 0.0557 68.9 1.03 0.98 3 50% CaX + 140.0 0.0339 58.0 1.16 1.03 50%NaX

[0036] The data in Table 4 indicate that the effective N₂ and O₂adsorption characteristics can also be altered by using highertemperature (above ambient) air feed to the VSA-O₂ system. This approachcan be used to obtain process performance which is very similar to thatfor LiX by using less expensive adsorbents. This option may beparticularly attractive because the currently required feed gas coolingstep after the air blower used in a single bed VSA system using a highlyselective nitrogen adsorbent can be eliminated.

[0037] Additionally, concepts described above in Tables 3 and 4 (i.e.,intimately mixed adsorbents and high feed temperature) can be used inconjunction with a simpler VSA cycle which eliminates the need for theuse of multiple beds. Thus, a single bed, single blower-vacuum pumpcombine can make the process simpler yet cost effective. The impact ofchanging materials and process design on the cost of O₂ product are notsignificant partly because (a) higher O₂ productivity generallyaccompanied by higher power which compensate each other and (b)adsorbents which give better performance are also more expensive whichneutralizes the performance advantage. Thus, less expensive commercialadsorbents can be used for competitive O₂ cost production when used witha discreet process design.

[0038] A concept for economic production of 80-95% oxygen from air wasdeveloped by using a VSA process cycle where a combination (physicalmixture or composite) of two or more nitrogen selective adsorbents(having low to moderate nitrogen adsorption selectivity and capacity)used in conjunction with a variety of process cycle designs and elevatedair feed gas temperatures. The net oxygen product costs from such asystem is lower than or equivalent to that obtained by using anexpensive high performance adsorbent, such as LiX, with high nitrogenadsorption selectivity and capacity.

[0039] The above described use of mixed adsorbents with or withouthigher feed air temperature in changing the effective shape of the N₂and O₂ isotherms can also be used in conjunction with simple (four-stepcycle) VSA-O₂ processes for lowering the cost of O₂ production. Theeight-step commonly used VSA-O₂ process requires at least two paralleladsorbent beds in tandem in order to provide the internal purge andpressurization gases. The simpler four-step VSA-O₂ process for producingan O₂ enriched product comprises: (a) feeding a feed gas (preferablyair) into a PSA or VSA bed; (b) countercurrent evacuation of said bed,(c) countercurrent purge of said bed with O₂ product under vacuum, and(d) simultaneous pressurization of said bed with O₂ product(countercurrent) and feed air (cocurrent). This simpler four-step cycleprocess can be operated using a single adsorber and a singlecombined-air blower and vacuum pump at a lower capital cost, becausethere are no steps where gas communication between two adsorbers areneeded.

[0040] Table 5 shows the comparative performance of the above describedsimpler four-step cycle VSA-O₂ using CaX zeolite at a feed airtemperature of 140° F. (60° C.). The O₂ production costs is verycompetitive with that for the commonly used VSA-O₂ process using LiXzeolite. TABLE 5 Relative O₂ Specific Feed Gas Productivity O₂ Power*Ex. Temperature (mlbmoles/ Recovery (KW/TP Relative No. ProcessAdsorbent (° F.) lb/cycle) (%) Cd) O₂ Cost 1 Commonly LIX 97.0 0.059770.0 1.00 1.00 Used VSA* (8step cycle) 2 Simple- CaX 140.0 0.0337 41.31.43 1.026 VSA** (4-step cycle) 3 Simple- 50% CaX + 97.0 0.0317 40.91.39 1.04 VSA** 50% NaX (4-step cycle) 4 Simple- 50% CaX + 140.0 0.029140.2 1.36 1.06 VSA** 50% NaX (4-step cycle)

[0041] The above demonstrates that N₂ and O₂ adsorption isotherms forair separation can be manipulated in various ways by using mixedadsorbents and/or using higher feed air temperature in order to producea cost effective O₂ product which is competitive with the commonly usedVSA O₂ process using LiX zeolite as the adsorbent. This is a surprisingresult. The flexibility created by the choice of pure or mixedadsorbents and operating conditions can also be utilized with a simplerfour-step cycle VSA-O₂ process which can be operated using a singleadsorber and a single air blower-vacuum pump combination which alsoresults in a competitive oxygen cost.

[0042] The above described results also show that many commerciallyavailable and less expensive adsorbents, such as NaX, CaX, and 5A, ortheir combinations in conjunction with many different VSA-O₂ processesof simpler designs and operating protocols can be used to produce an O₂product which competes well with the commonly used VSA-O₂ process costs.

[0043] The present invention has been set forth with regard to severalpreferred embodiments, however the full scope of the present inventionshould be ascertained from the claims which follow.

1. A process for producing an oxygen enriched product from a feed gascontaining oxygen and nitrogen, said process comprising: providing a gasseparation apparatus comprising at least one bed containing a physicalmixture of at least two different nitrogen selective adsorbents, whereinsaid at least one bed is free of lithium cations; feeding said feed gasinto said gas separation apparatus to contact said at least one bed; andrecovering from said gas separation apparatus said oxygen enrichedproduct.
 2. The process according to claim 1, wherein said feed gas isair.
 3. The process according to claim 1, wherein said mixture is acomposite of adsorbent particles or mixture of independent adsorbentparticles.
 4. The process according to claim 1, wherein said mixture isa homogeneous mixture of adsorbent particles.
 5. The process accordingto claim 1, wherein one of said two different nitrogen selectiveadsorbents is a zeolite.
 6. The process according to claim 5, whereinsaid zeolite contains a monovalent cation or a divalent cation.
 7. Theprocess according to claim 1, wherein said two different nitrogenselective adsorbents are two different members selected from the groupconsisting of sodium, calcium, barium, strontium, magnesium, cesium,exchanged X zeolite, exchanged A zeolite, and mordenite zeolite.
 8. Theprocess according to claim 1, wherein said mixture contains from about20% to about 80% by weight of a first of said two different nitrogenselective adsorbents, and from about 80% to about 20% by weight of asecond of said two different nitrogen selective adsorbents.
 9. Theprocess according to claim 7, wherein said mixture contains about 50% byweight of each of said two different nitrogen selective adsorbents. 10.The process according to claim 1, further comprising providing said feedgas at a temperature above ambient.
 11. The process according to claim10, wherein said temperature is from about 40° C. to about 100° C. 12.The process of claim 1, further comprising: countercurrently evacuatingsaid at least one bed following said feeding; countercurrently purgingsaid at least one bed with said oxygen enriched product under vacuum;and simultaneously pressurizing said at least one bed with acountercurrent stream of said oxygen enriched product and a cocurrentstream of said feed gas.
 13. The process according to claim 12, furthercomprising providing said feed gas at a temperature above ambient. 14.The process according to claim 13, wherein said temperature is fromabout 40° C. to about 100° C.
 15. A process for producing an oxygenenriched product, said process comprising: providing a gas separationapparatus comprising at least one bed; feeding a feed gas containingoxygen and nitrogen into said gas separation apparatus to contact saidat least one bed, wherein said feed gas is at a temperature aboveambient; countercurrently evacuating said at least one bed followingsaid feeding; countercurrently purging said at least one bed with saidoxygen enriched product undervacuum; simultaneously pressurizing said atleast one bed with a countercurrent stream of said oxygen enrichedproduct and a cocurrent stream of said feed gas; and recovering fromsaid gas separation apparatus said oxygen enriched product.
 16. Theprocess according to claim 15, wherein said temperature is from about40° C. to about 100° C.
 17. The process according to claim 13, whereinsaid at least one bed contains a mixture of at least two differentnitrogen selective adsorbents, and is free of lithium cations.
 18. A gasseparation apparatus adapted to perform the process of claim
 1. 19. Thegas separation apparatus of claim 18, comprising a single adsorber and asingle combined air blower-vacuum pump.